The present invention relates to an automotive radio receiver incorporating a radio frequency interference elimination circuit. More specifically, the invention relates to a radio frequency interference elimination circuit for an automotive radio receiver for eliminating noise in radio output sound created by the ignition system of an automotive engine and other automotive devices.
In an automotive vehicle, radio frequency interference can be produced from many sources such as a spark ignition noise. This radio frequency interference causes noise in the audio output of the radio receiver. Vehicular radio frequency interference is particularly intense in the AM radio band.
Radio frequency interference in an automotive radio receiver is mainly due to sharply-peaked noise signals created by the spark ignition system, alternater switching and so on. For suppressing such radio frequency interference, various systems have been proposed. For example, Japanese Patent Pre-Publication (Tokkai) Sho. No. 54-98509, pre-published on Aug. 3, 1979 illustrates a radio frequency interference suppression system having a gate circuit inhibiting reception of the broadcasting signal while noise is being generated and a hold circuit for holding the receiving signal level immediate before the noise starts.
However, the conventional radio frequency interference suppression system can not satisfactorially and successfully suppress noise in the audio output thereof. In the foregoing specific prior art, waveform distortion can be caused by holding the signal level while the noise is being generated.
It is easy to detect relatively intense radio frequency interference radiation such as ignition noise by providing a noise detector antenna in the engine room. However, even with such a noise detector antenna, it is difficult to detect relatively weak radio frequency interference such as switching noise in the voltage regulator or the vehicle accessories.